Water quality is often indicated by the total organic carbon (TOC) present in the sample. Total organic carbon (TOC) is a well-established water quality parameter that quantifies the overall concentration of organic substances, all of which are typically regarded as contaminants. In most aqueous samples, such as drinking water, raw water, wastewater, industrial process streams, and the like, the total carbon (TC) is the sum of the amount of total organic carbon (TOC) and the amount of inorganic carbon (IC) present in the sample.
Most TOC measurement techniques involve 3 steps: first, measure the concentration of inorganic carbon (i.e. carbon in all the forms of dissolved carbon dioxide); second, oxidize organics in sample water to CO2; and third, measure the CO2 derived from the organics. Some instruments, however, allow for the acidification of the sample and purge CO2 so that the inorganic carbon (IC) concentration is not measured. Oxidation of the organics in the water sample is traditionally achieved by UV radiation (with or without a chemical oxidizer present), combustion, or heat treatment (with or without catalysts or oxidizing agents). Mercury vapor lamps used in UV radiation cause harmful radiation and have a short service life. UV-persulfate oxidation adds harmful chemical oxidants.
Use of electrochemical methods has become an attractive alternative to traditional methods for treating water that contains dissolved organic compounds. Generally, organic pollutants dissolved in the water can be destroyed electrochemically by direct anodic oxidation at the electrode surface or indirectly through oxidation processes mediated by electrogenerated oxidants. The compound's oxidation potential and the choice of electrode material both influence where oxidation is by direct or indirect means. Classical electrode materials like platinum tend to suffer from several problems when used during electrolytic oxidation. Slow reaction rates, low efficiencies, and deactivation of the surface are possible. Corrosion during anodic polarization may also occur. Platinum offers a limited anodic range making direct organic oxidation problematic.
Conventional methods to measure inorganic carbon (CO2, H2CO3, HCO3−, and CO32−) and the CO2 obtained from oxidation of the organics include conductivity measurements, infrared absorbance photometry, reaction of CO2 with a colored indicator and measurement of the intensity of that color or CO2 conversion to another species followed by flame ionization detection.
Conductivity detectors measure CO2 in the water sample and may be divided into two groups: direct and membrane-based conductivity. The direct conductivity method is susceptible to interference from the ionic content of water from other sources besides CO2. In the membrane-based conductometric method, a CO2— permeable membrane is located between the sample water chamber and the deionized water acceptor chamber to separate the CO2 from the sample matrix. The membrane selectively passes only CO2 and serves as a protective barrier to interfering substances in the liquid.
Most TOC analyzers that operate based on combustion oxidation and conventional measurement techniques are typically, large, complex, and costly instruments. Many TOC analyzers are laboratory units. These units are not portable or suitable for “field use”, wherein the instruments are brought to the water system being tested and are used to test the system for a short period of time. Instead, samples must be brought to the analyzer for testing in a controlled environment. Many TOC analyzers are not easily adapted to an “on-line” system wherein the instrument is placed in the water system being tested and aqueous samples may be tested and monitored automatically, without human intervention.
TOC analyzers may also be classified as “flow-through” or “batch” instruments. The term “flow-through” is used to describe an instrument wherein the samples are flowing samples streams as opposed to “batch” wherein the samples are collected and analyzed. In flow-through instruments, the sample may flow continuously through the instrument as it is analyzed and may be returned to the sample source or directed elsewhere for treatment or disposal. Many flow-through instruments, however, may also have an auto sampler wherein the samples are collected and analyzed, allowing the instrument to operate as a batch instrument.